Automatic gain control of an audio system



May 16,11950 w. w. H. DEAN v AUTOMATIC GAIN CONTROL OF AN AUDIO SYSTEM Filed July 23, 1947 LA 7 w w.\n@/4 T fd W V/ A u A A A F Y www. 1 E M .mg f M# F .a mw w M .0 A m# 1:.. i @u fa ww l rf 0/ Hm@ www, HF Wa/n/ UMa Fly lIl]

ATTORNEY source B3 more negative than BI.

Patented May 16, 1950 AUTOMATIC GAIN CONTROL OF AN AUDIO SYSTEM William W. H. Dean, Pointe Claire, Quebec, Canada, assignor to Radio Corporation of America, a corporation of Delaware Application July 23, 1947, Serial No. 762,951

10 Claims. l

This application concerns audio amplifiers of controllable gain such as are used in speech prou gram amplification for various uses such as modulation of carrier currents.

The primary purpose of my invention is to provide a simple circuit for instantaneously adjusting the gain of an audio amplier with varying speech input so as to maintain its peak output constant.

In describing my invention in detail reference will be made to the attached drawings wherein I illustrate schematically and by block diagram in Figure l and by block diagram and circuit element and circuit element connections in Figure 2 a variable gain amplifier automatically controlled as to gain in accordance with my invention.

In the drawings, IE represents the variable gain amplifier having an input at leads I2, and an output at leads I4. The variable gain amplifier is controlled as to gain by a biasing potential which is developed on lead I6 and applied to one or more tube stages in the amplifier. This potential operates as grid bias on amplier tube stages in a manner too well known to warrant description herein.

'I'he variable gain amplier supplies output through lead 20 to an amplifier 22 referred to hereinafter as amplifier #I and to a current amplitude clipper 24 used herein as a reference source. The clipper 24 has a constant amplitude output as indicated by the wave form above the lead and serves as a reference source as described hereinafter. The storage condenser which supplies the bias is designated C3 and is shunted by variable resistor R3 which slowly restores a fixed negative potential BI to #I terminal of C3 in the absence of signal. The storage condenser has a discharge path through the impedance of tube VI which has its anode connected to the positive terminal of a source B5 and its cathode connected to the upper end of capacitor C3 and the anode oi a tube V2 whose cathode is connected to a negative terminal of a The tube V2 is in the condenser charging path since it permits a negative potential on the #I condenser terminal to build up. The tube VI is excited by output from the current amplitude clipper 24 after amplication in amplier 2F, herein designated as #2 amplier. This excitation from amplifier #2 is by way of lead 28 and capacitor CI. Tube yVI is biased by output from amplifier #I rectified in rectier 30 herein designated as reetier #L This bias path includes the resistor RI.

The tube V2 is excited by output from amplifie #I by lead 34 through capacitor C2 and derives its bias from rectiiier 36 excited by the output of amplifier 26 and connected by resistor R2 to the grid of tube V2.

Referring to the drawing, a variable gain ampliiier l0 is provided which is biased by the voltage appearing across capacitor C3. This bias is made to vary according to the peak signal level and thus adjust the gain of the amplifier to maintain constant peak amplitude of the output signal of Il). The voltage on capacitor C3 is made to vary by varying the conductivity of tubes VI and V2 in the following manner. With no input signal, both tubes VI and V2 are biased beyond cutoff by voltages BI and B2 respectively. The voltage across capacitor C3 slowly varies due to resistor R3 until the voltage on terminal I of capacitor C3 approaches BI. The resulting negative voltage on terminal I of capacitor C3 reduces the gain of the variable gain amplifier I0 to a minimum during idle periods. Now, when a signal is applied at the input, it is amplified in the main amplifier III and in amplier #L rectified in rectier #L poled so that negative potential is developed on the grid of VI, so that raising the gain of the main amplier IIJ operates to increase the negative bias on the tube VI. At the same time signal amplified in the main amplier is clipped at 24 and amplified in #2 amplifier. The clipped output signal of amplin er #2 is applied to tube VI grid. The peak output signal of the clipper is constant and is adjusted to be greater than and overcome the bias, produced in rectifier #I on the grid of tube VI by the signal when the gain of the variable amplifier is minimum. The result is current ilow in tube VI to raise its cathode potential and cause a decrease in negative bias on the main amplier. The voltage on capacitor C3 varies in the positive direction to increase the gain of the main amplifier until the bias on tube VI (rectified current in #I rectier) has increased sufficiently to prevent the clipped signal amplified in #2 amplifier from overcoming the said bias thereby preventing conduction in tube VI on clippedsignal peaks. During this action, tube V2 has been biased negative by the clipped signal, rectified in #2 rectier, and has had the amplified output signal from amplifier #I applied to its grid. As the peak clipped signal, rectied in #2 rectifier, has exceeded the peak unclipped signal the grid of tube V2 has remained cut off. If, however, the signal increases further, causing the unclipped peak to exceed the clipped peak then tube VI remains cut oiI, tube V2 conducts to provide a charging path for capacitor C3 and the voltage on capacitor C3 moves in the negative direction. The variable amplifier gain decreases until equilibrium is again restored.

From this it may be seen that the variable amplifier gain will be varied until the peak value of the output wave equals the peak of the clipped wave. This latter is a xed value and serves as a reference. The clipper need not have suiicient sensitivity to start clippingV when the weakest input signal is applied to the input when the variable amplifier is at minimum gain as the clipper output amplitude will exceed the input amplitude hence insuringthat Vl conductsto increase the variable amplifier gain as already described.

The minimum gain of the ampliiier, as previously mentioned, is set at the point at which tube VI is biased to cut off in the absence of signal. Toprevent the'ampliil'er from attempting to'reach' an excessive gain, a'dicde V3, biased negative to a potential B4, isfplaced across capacitor C3. This limits the minimum negative bias on the variable gain amplierto a safe value.

Resistor R3 maylbe varied to adjust thedecay time ofthe amplifier gain. If this decay'is made relatively fast, it will minimize background noise by, for example, reducing the gain between words or in pauses in speech originating in a noisy location. On the other hand' by eliminating R3 entirely the gain would remain at the last operating level making this device operate as a voice operated gain adjusting=device such as is used in telephone practice.

The apparatus used in unitsv 10, 22, 24 andV 26 of TFigure 1 may be' as known in the art and will not be described herein: The rectiers 3G and 36 andtheir connections to the amplifiers andcapacitor have been shown in Figure 2. The rectiers #I and #2 may be diodes 30 and 36' in rectier circuits includingl load impedances `3| and 3l across which the rectiiied currents develop potentials the magnitude of whichis Vproportional to the V'current intensity'. The developed' potentials are fed a's bias; by resistorsRl 'and R2 respectively,l to the controlgridsf of tubes VI and `V2. The opera-tion is asdescribed 'in'V connectionv with Figure-1 andwill not be' repeated-here. A source to supply` the rvarious direct current potentials used may take the formof'a potential Idivider PD-acrcss a'direct current source .as'shown in Figure 2. A point intermediate theY ends of PD is groundedl so that B4; BI, BS-and B2 are negative Vand increase in magnitudefromthe grounded point to reach a maximumf'value at B2. B5 is positiveand is taken from a: point-between -two resistors in series across one'tube of two voltage regulator tubes V6 and V'l in series-across the direct current source.

The advantage of -such an amplieris apparent inradio communication systems for maintaining high modulation levels. The-present circuit is much less complicated-than thefvoice operated gainadjustment device used'iatelephone systems for theifsame purpose. A'further advantage is that a clipped output-maybe obtained'at the 20utput forusefin increasingfthe' average modulaticnpf'a carrier as described morel in detail in my U. S. application Serial'Number 758,693, l'ed July 2,1947. Here, as in said application, noise vis suppressed inthe absence of signals becausethe'outputof the mainamplier-'is reduced to a point below 'that at which 2d-startsfclipping.

stage 42 of said application. The voltage ampliiler and phase reverse portion of said application may then be of variable gain and may replace amplifier I9 of this application. Furthermore, by proper choice of gains, the degree of clipping may be maintained at the optimum value over a considerable range of inputs.

What is claimed is;

1. In apparatus for automatically holding the amplitude of signals at the desired level, a variable gain amplier having an input excited by signal voltage and having an output wherein the amplied voltage appears, a storage capacitor across which a potential is developed and fed to said amplier to control its gain, means for developing voltage of fixed level only when said signal voltage excites said amplier, two electron discharge tubes each having a control grid and having their` internal impedances in series across a potential source, a connection between a point on said series connection, intermediate the tube impedances, and said capacitor, means for applying a bias to one grid which is a measure of the average magnitude of the ampliers output, means for `applying said voltage of fixed level from said nist-named means to said last named grid, means for applying a bias to the other grid which is a measure of the average magnitude of said voltage of xed level, and means for applying voltage from said amplifier to said last named grid.

2. Apparatus as recited in claim 1, including a source of potential and a resistor connected to said capacitor to place a base charge thereon, in the absence of signal voltage excitation of said amplier of variable gain, after a time interval measured by the magnitude of said resistor.

3. Apparatus as recited in claim 1 wherein said first-named means is a limiting amplifier excited by the signal voltage.

4. Apparatus as recited in claim l wherein said capacitor is shunted by a diode to limit the charge on said capacitor,

5. In apparatus for automatically holding the amplitude of signals at the desired level, a variable gain amplifier having an input excited by Vsignal voltage and having an output wherein the amplified voltage appears, a capacitor across which a potential is developed and fed to said amplifier to control its gain, a voltage clipper excited by the signal voltage ior developing voltage of ixed level only when said signal voltage excites said amplifier, two electron discharge tubes each having a control grid and having their internal impedances in series across a potential source, a connection between a point on said series connection, intermediate the tube impedances, and said capacitor, a rectifier excited by signal voltage for applying a bias to one grid which is a measure of the average magnitude of the ampliers output, connections for applying said clipper voltage of xed level from said clipper to said last named grid, a rectiner excited by voltage from said clipper for applying a bias to the other grid which is a measure of the average magnitude of said voltage of fixed level, and connections for applying amplied voltage from said amplier to said last named grid.

6. Apparatus as recited in claim 5 including a resistor and a source of direct current potential connected to said capacitor to place a base charge thereon.

7. Apparatus as recited in claim 5 including a charge limiting diode in shunt to said capacitor.

8. In apparatus for automatically holding the amplitude of signals at a desired level. a variable Theclipper 24l per seamay.` belike'the clipper 75 gain amplifier having an input excited by signal asovgees voltage and having an output wherein the ampliiled signal voltage appears, a capacitor the potential across which is fed to said amplifier to control its gain, means coupled to the output of said amplier for producing from the amplified signal voltage a voltage of ixed level, means responsive to the output of said amplier and to said voltage of fixed level for producing a potential when the amplier output voltage is above said xed level, other means responsive to the output of said amplier and to said voltage of fixed level for producing a potential when the amplier output voltage is below said xed level, said produced potentials varying in opposite directions with respect to a reference potential, and connections for impressing said produced potentials on said capacitor.

9. In apparatus for automatically holding the amplitude of signals at a desired level, a variable gain ampliiier having an input excited by signal voltage and having an output wherein the amplied signal voltage appears, a capacitor the potential across whicli is fed to said amplifier to control its gain, means coupled to the output of said amplifier for producing from the amplified signal voltage a voltage of xed level, means responsive to the output of said amplier and to said voltage of fixed level for producing a potential when the amplifier output voltage is above said fixed level, other means responsive to the output of said arnplifler and to said voltage of xed level for producing a potential when the amplier output voltage is below said fixed level, said produced potentials varying in opposite directions with respect to a reference potential, connections for impressing said produced potentials on said capacitor, and

means for establishing a selected potential across said capacitor in the absence of signal voltage excitation of said amplifier.

10. In apparatus for automatically holding the amplitude of signals at a desired level, a variable gain amplifier having an input excited by signal voltage and having an output wherein the amplifled signal voltage appears, a, capacitor the potential across which is fed to said amplier to control its gain, means coupled to the output of said amplilier for producing from the amplified signal voltage a voltage of fixed level, means responsive to the output of said amplifier and to said voltage of fixed level for producing a potential when the amplifier output voltage is above said fixed level, other means responsive to the output of said amplifier and to said voltage of xed level for producing a potential when the amplifier output voltage is below said flxed level, said produced potentials varying in opposite directions with respect to a reference potential, connections for impressing said produced potentials on said capacitor, and a resistor and a potential source connected across said capacitor for establishing a selected potential thereacross in the absence of signal voltage excitation of said amplifier.

WILLIAM W. H. DEAN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,316,354 Moritz, Jr Apr. 13, 1943 2,402,445 Poch June 18, 1946 

